Internet Protocol (IP) Multimedia Subsystems (IMS) refers to a core network technology related to ‘A11 IP’ which integrates (unifies) wired and wireless communication environments. The IMS was introduced by a Third Generation Partnership Project (3GPP) which develops global specifications of a wireless communication. The IMS adapts an infrastructure for providing an IP multimedia service.
An IMS based service aims to compositively provide multimedia data such as voices, audio, video, data and the like based upon the IP protocol, and to construct an infrastructure in which services can rapidly be developed and varied.
An introduction of an A11 IP based network through the IMS is mainly directed to receiving abundant contents and new service capabilities which are provided using a packet service into an integrated network.
A logical structure of the A11 IP network is roughly classified into a radio network domain, a GPRS based packet switched service domain and an IP multimedia service domain. Explanations will be given for each construction of the A11 IP network as follows.
First, the radio network domain is constituted with a mobile terminal which refers to a service user, a node B managing a connection of the mobile terminal via a wireless interval, and a node such as a Radio Network Controller (RNC) and the like,
The GPRS based packet service domain is constituted with a serving GPRS support node (SGSN) and a gateway GPRS support node (GGSN) for managing a user packet data delivery between a wireless access network domain and an external network or an Instant Message (IM) service domain, managing a mobility of the mobile terminal and providing a PDP content activation service.
FIG. 1 illustrates a structure of an IMS domain communication network.
As illustrated in FIG. 1, the IMS domain is constituted with a Serving-Call Session Control Function (S-CSCF) for managing a multimedia call processing function and a registration using a SIP protocol developed by an Internet Engineering Task Force (IETF), and a Home Subscriber Server (HSS) for implementing a Home Location Register (HLR) function of a related art mobile network and a function related to an IP multimedia user s mobility and to authentication, the two functions being integrated with each other. Besides, the A11 IP network may further include a Media GateWay (MGW) for controlling a media gateway to thus interwork a signaling and a call control with a related art PSTN network, for supporting multimedia to thus control the GGSN and bearers for multiple multimedia conference services, and for managing a circuit bearer and a packet media stream channel.
Functions defined in the IMS domain in the logical structure of the A11 IP network will be explained with reference to FIG. 1.
FIG. 2 illustrates a schematic structure of functions of an IMS domain.
As illustrated in FIG. 2, the IMS domain may includes a Proxy Call Session Control Function (P-CSCF), an Interrogation Call Session Control Function (I-CSCF) and a Service Call Session Control Function (S-CSCF). First, the P-CSCF denotes a first access point at which the user accesses the IMS network, and exists in same domain as one the GGSN exists in. The P-CSCF acts as a proxy and a user agent and transfers a SIP registration request message received from the user to the I-CSCF with reference to the user's home domain. The P-CSCF also transfers a SIP message received from the user to the S-CSCF using an S-CSCF address received by a registration procedure, and then requests the SIP message from the user or responses to the user with respect to the SIP message. In order to use the IMS service, a session setup is required via a negotiation process for transmitting and receiving capabilities and preferences among end users using the SIP. Once setting the session, actual bearers are established among the end users terminals to thus enable a data exchange among the end users.
In addition, the IMS uses a SIP/SDP protocol provided by the IETF to thus manage the session. That is, the IMS can support an end-to-end multimedia service based upon the SIP. Hereinafter, the SIP/SDP protocol will now be explained in more detail.
The SIP denotes a signaling protocol of an application level defining procedures for finding locations of each of intelligent terminals which are to communicate together on an Internet by identifying each of them, and generating, deleting and changing a multimedia communication session among the intelligent terminals. In the communication using the SIP, a caller sends a message in a text format to a callee in order to generate (set) a new session with the callee or to make the callee participate in a previously established session. The setup session may substantially include contents which are described in one or more media formats such as audio, video, text or white board, to therefor use an Internet Protocol (IP) referred to as a Session Description Protocol (SDP).
TABLE 1Type of messageValueFunctionINVITE1xxInformationACK2xxSuccessBYE3xxRedirectionCANCEL4xxClient ErrorRESIGER5xxServer ErrorOPTION6xxGlobal Error
Table 1 indicates types of the SIP messages and functions thereof. As shown in Table 1, each type of the messages is divided based upon a first digit (e.g., 1xx for the INVITE, and 4xx for the CANCEL) of a value corresponding to the message. All of the SIP message is composed in a text format. Upon sending a particular message, one TCP segment or a UDP datagram allows several messages to be sent by use of a Transmission Control Protocol (TCP) or a User Datagram Protocol (UDP).
FIG. 3 illustrates a format of a SIP message, and FIG. 4 illustrates an example of a SIP message using the SIP message format shown in FIG. 3. As illustrates in FIGS. 3a and 3b, ‘METHODS’ indicates a message starting line for describing a type of message (i.e., INVITE in FIG. 4), a request ID (i.e., 128.16.64.09/65729 in FIG. 4), and a SIP version (i.e., SIP/2.0 in FIG. 4). ‘HEADER’ of the SIP is composed of one or more addresses of nodes routed (i.e. path), addresses of sender and receiver, and sequential number and TTL indicating the number of times being connected to the same address. In order to define the PATH value included in the header within a network, the message should be able to be modified in a message sending process, which causes a security related problem with respect to the message. ‘BLANK LINE’ describes a boundary between the header and a message body. ‘MESSAGE BODY’ indicates a place for receiving and sending a content of the message and an attribute value required for a communication.
A negotiation process for setting up (establishing) the session between originating end and terminating end through the SIP message in the format shown in FIG. 3 is disclosed in IMS Specification TS 23.228, that is, in which end users (i.e., users of the originating and terminating ends) have different capabilities and preferences depending on end users communication states (e.g., depending on who has requested the session setup, or the current state the terminating end is faced with, or when media format requested by the originating end is different from one preferred by the terminating end. In such cases, in order to set a more satisfactory session, provided is a section with respect to a capability which the originating end prefers and a capability which the originating end can currently support. Accordingly, the terminating end can accept or reject an incoming session in consideration of the current communication state. The originating end can use the SIP/SDP protocol to thus request the session setup from the terminating end by including in a particular parameter media formats (e.g., audio, video, text, white board, etc.) and a codec which the originating end intends to use together with the terminating end through the corresponding session. The terminating end selects or rejects acceptable capabilities (e.g., media formats, codecs, etc.) according to the protocol.